When vinyl chloride or a mixture of vinyl chloride and a copolymerizable monomer, such as vinyl acetate, is polymerized in an aqueous medium, there is obtained an aqueous dispersion that contains from 5% to 50% by weight of a vinyl chloride resin and up to 5% by weight of vinyl chloride. Most of the unreacted vinyl chloride is usually removed by heating the aqueous dispersion under subatmospheric pressure to 65.degree. C. As it is ordinarily practiced, this stripping procedure reduces the monomer content to about 1000 ppm to 15,000 ppm. Further processing yields dried products that contain 500 ppm or more vinyl chloride.
In view of the recently developed safety standards that require that the amount of vinyl chloride in vinyl chloride resins and in the atmosphere be maintained at very low levels, it is necessary that the monomer content of the aqueous dispersions be sufficiently reduced that these requirements can be met.
A number of procedures for the removal of vinyl chloride from aqueous dispersions of vinyl chloride resins have been reported, but none has proven to be entirely satisfactory. Procedures that involve heating the dispersions to temperatures above 65.degree. C. for prolonged periods or subjecting them to conditions of high shear are effective in reducing the vinyl chloride content to low levels, but they usually have an adverse effect on the stability and filterability of the dispersions and on the color and other physical properties of the products. Among the reported procedures that involve the heating of aqueous dispersions of vinyl chloride resins to remove residual monomer from them are those disclosed in U.S. Pat. Nos. 2,820,028, 3,975,230, 4,017,445, 4,020,032, and 4,086,414 in which the aqueous dispersions are contacted with steam at temperatures above 80.degree. C. to remove vinyl chloride from them and that disclosed in U.S. Pat. No. 4,086,412 in which a polyvinyl chloride-containing slurry is treated with hot water to reduce its monomer content. Procedures in which the aqueous dispersions are contacted with organic compounds either do not reduce the monomer content to the desired very low levels or they do so too slowly to be practiced commercially. Among the organic compounds that have been used in these processes are aliphatic hydrocarbons such as gasoline (U.S. Pat. No. 3,052,663), alcohols and/or epoxy compounds (U.S. Pat. No. 3,847,853), dioctyl phthalate and other plasticizers (U.S. Pat. No. 4,001,482), and vinyl acetate, methylene chloride, and di-tert.butyl-p-cresol (U.S. Pat. No. 4,015,064).